The subject matter disclosed herein relates to an X-ray CT (Computed Tomography) apparatus, and particularly to an X-ray CT apparatus for performing X-ray CT imaging on the same anatomical region in a subject with a plurality of kinds of X-rays having different energy distributions to obtain a plurality of kinds of tomographic images representing the same slice but having different image properties.
Conventionally, there is a known X-ray CT imaging method comprising performing X-ray CT imaging on the same anatomical region in a subject with two kinds of X-rays having energy distributions different from each other to obtain two kinds of tomographic images representing the same slice but having image properties different from each other, and performing inter-image computational processing using the two kinds of tomographic images to obtain an enhanced image in which a difference between the tomographic images is enhanced (see Patent Document 1, for example).
In general, different kinds of substances have different X-ray energy absorption distributions, and when a subject is imaged with two kinds of X-rays having different energy distributions, the tomographic images of the subject obtained from the imaging operations have image properties, for example, contrasts, different from each other. Therefore, by focusing upon such a difference between the tomographic images, an image representing only a given substance or tissue can be extracted to aid in interpretation of tomographic images.
When imaging a subject with two kinds of X-rays to obtain such an enhanced image, a time difference occurs between the first and second imaging operations in principal, and a change in the position or posture of the subject between the first and second imaging operations causes a subject's positional offset between the two tomographic images. Such a positional offset causes incorporation of artifacts, in addition to a true difference due to the difference in image property between the tomographic images, into the enhanced image, thus interfering with proper observation or analysis of the enhanced image.
While it is possible to restrain subject's conscious action or respiration to some degree with the cooperation of the subject, a positional offset of the subject involved in, especially, subject's cardiac motion cannot be restricted. Thus, for example, when X-ray CT imaging is performed with an subject's artery taken as an anatomical region of interest, the position of the artery itself may vary with cardiac motion or the thickness of the artery may vary with a change of the blood flow rate, resulting in a positional offset of the anatomical region of interest among a plurality of kinds of tomographic images.
A method for solving such a problem that may be contemplated comprises applying an electrocardiological synchronized imaging technique. The electrocardiological synchronized imaging technique includes a prospective imaging method and a retrospective imaging method. The prospective imaging method comprises monitoring the subject's electrocardiographic waveform by an electrocardiograph, etc., and causing each scan to synchronize with a given cardiac phase after a given period of time from an R peak (see Japanese Patent Application Laid Open No. 2006-6531, for example). The retrospective imaging method comprises collecting projection data simultaneously with electrocardiographic waveform data of a subject, and extracting only the projection data corresponding to an arbitrary cardiac phase later in image reconstruction (see Japanese Patent Application Laid Open No. 2003-164446, for example). Since by applying such electrocardiological synchronized imaging techniques, reconstruction processing may be performed using projection data corresponding to a given cardiac phase zone in which subject's motion associated with cardiac motion is slow, the subject's positional offset due to cardiac motion among the resulting tomographic images can be suppressed.